**Acknowledgements**

*Earth Crust*

**Figure 18.**

activation of the fan mechanism, the lithospheric strength is determined by friction. However, if the fan mechanism is activated somewhere causing the new rupture development in intact rock, the transient lithospheric strength in that region decreases to the level τfan. After completion of the failure process, the lithospheric

*Illustration of depth distribution for rock transient strength, brittleness and earthquake frequency in the Earth's* 

It should be noted that the improved concept of the lithospheric strength incorporates all three types of rock strength determining the instability in the seismic layer: fracture strength τs, frictional strength τf and fan strength τfan. The fracture strength determines the level of local stress at which the initial fan structure can be generated. The shaded area between the frictional τf and the fan-transient τfan profiles determines levels of field stress under which an initiated fan structure can propagate creating an earthquake. Importantly, the fan mechanism can cause earthquakes at any level of field stress τ within the shaded zone. Due to this the highest probability of events is at a depth characterised by the maximum range between τfan and τf. This depth corresponds approximately to the depth of optimal efficiency of the fan mechanism, where the rock mass is characterised by the minimum transient strength and maximum brittleness. At lower and greater depths, the probability decreases. This feature determines the typical depth-frequency distribution of earthquake hypocentres. The upper and lower cut-offs represent boundaries of the zone of the fan-mechanism activity. The explanation for the depth distribution of earthquake frequency on the basis of the fan mechanism differs fundamentally

On the basis of the fan mechanism, it is possible also to explain the existence of a few zones of earthquake activity with depth. As discussed in **Figure 12**, the efficiency of the fan mechanism depends on the rock hardness (UCS): the harder the rock, the greater the fan-mechanism efficiency and the wider the confining pressure range over which the fan mechanism is active. **Figure 18a** illustrates schematically depth distributions of the fan-mechanism activity for four rocks characterised by different hardness, with strength increasing from rock 1 to rock 4. **Figure 18b** shows a situation when the earth's crust is represented by two layers of rocks of different hardness (rock 1 and rock 3). In this case, two zones of earthquake activity may be observed. Rock 1 will exhibit the typical (complete) form of earthquake

strength returns to the frictional strength.

*crust represented by two layers of rocks of different hardness.*

from the conventional explanations.

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This work was supported by the Ministry of Science and Education of the Russian Federation (grant no. RFMEFI58418X0034).
